Nowadays, steel materials for welded structures are required to have not only high strength and high toughness, but also a low yield ratio and high uniform elongation from the viewpoint of earthquake resistance. Generally, it is known that, by forming a metallographic structure of a steel material in which hard phases such as a bainite phase (hereinafter, also referred to as β) and a martensite phase (hereinafter, also referred to as M) are appropriately dispersed in a ferrite phase (hereinafter, referred to as α) which is a soft phase, it is possible to achieve a decrease in the yield ratio of the steel material and an increase in the uniform elongation of the steel material.
As an example of manufacturing methods of forming a microstructure in which hard phases are appropriately dispersed in a soft phase as described above, a certain method is described in Japanese Unexamined Patent Application Publication No. 55-97425. That is, JP '425 discloses a heat treatment method in which, as an intermediate treatment between quenching (hereinafter, also referred to as Q) and tempering (hereinafter, also referred to as T), quenching starting from a temperature range for forming a dual phase consisting of a ferrite phase and an austenite phase (hereinafter, also referred to as γ) is performed (hereinafter, also referred to as Q′).
Japanese Unexamined Patent Application Publication No. 55-41927 discloses an example of a method which does not require an additional manufacturing process, in which, after rolling has been finished at a temperature equal to or higher than the Ar3 transformation point, the start of accelerated cooling is delayed until the steel material has a temperature equal to or lower than the Ar3 transformation point at which a ferrite phase is formed.
As an example of a technique with which it is possible to achieve a decrease in yield ratio without performing complicated heat treatments as disclosed in JP '425 and JP '927, Japanese Unexamined Patent Application Publication No. 1-176027 discloses a method with which a decrease in yield ratio is achieved by finishing rolling of a steel material at a temperature equal to or higher than the Ar3 transformation point, and then by controlling an accelerated cooling rate and a cooling stop temperature to form a dual-phase structure consisting of an acicular ferrite phase and a martensite phase.
Moreover, Japanese Patent No. 4066905 discloses an example of a technique with which it is possible to achieve a low yield ratio and excellent toughness in a weld heat affected zone without significantly increasing the contents of alloying chemical elements of a steel material, in which, by controlling Ti/N and a Ca—O—S balance, a three-phase structure consisting of a ferrite phase, a bainite phase and island martensite (hereinafter, also referred to as MA) is formed.
In addition, Japanese Unexamined Patent Application Publication No. 2008-248328 discloses a technique in which a decrease in yield ratio and an increase in uniform elongation are achieved by adding alloying chemical elements such as Cu, Ni and Mo.
On the other hand, in a welded steel pipe used to form a linepipe such as a UOE steel pipe or an electric resistance welded steel pipe, after a steel plate has been subjected to cold forming into a cylindrical shape and the butted portions have been welded, usually, a coating treatment such as polyethylene coating or powder epoxy coating is performed on the outer surface of the resultant steel pipe from the viewpoint of, for example, corrosion protection. Therefore, there is a problem in that, since strain ageing occurs due to working strain applied when pipe forming is performed and due to heating when the coating treatment is performed, there is an increase in yield stress, which results in the yield ratio of the steel pipe being larger than that of the steel plate.
To solve the problem described above, for example, Japanese Unexamined Patent Application Publication Nos. 2005-60839 and 2005-60840 disclose steel pipes having a low yield ratio, high strength and high toughness excellent in terms of strain ageing resistance and methods of manufacturing the steel pipes utilizing the fine precipitations of complex carbides containing Ti and Mo or the fine precipitations of complex carbides containing two or all of Ti, Nb, and V.
In the heat treatment method according to JP '425, by appropriately selecting a quenching temperature in a range forming a dual phase, it is possible to achieve a decrease in yield ratio, but there is a problem in that there is a decrease in productivity and there is an increase in manufacturing cost due to an increase in the number of heat treatment processes.
In addition, in the technique according to JP '927, there is a problem in that, since it is necessary to perform cooling at a cooling rate equivalent to that of a natural cooling in a temperature range from a rolling finish temperature to an accelerated cooling start temperature, there is a significant decrease in productivity.
Moreover, in the technique according to JP '027, as indicated by the examples in the literature, there is a problem in that, since the carbon content or the contents of other alloying chemical elements of a steel plate are increased to obtain a steel material having a tensile strength of 490 N/mm2 (50 kg/mm2) or more, there is an increase in material cost, and, in addition to that, there is a decrease in toughness in a weld heat affected zone.
In addition, in the technique according to JP '905, the influence of, for example, a microstructure on uniform elongation which is required in use for, for example, a linepipe has not necessarily been clarified. In addition, since the low-temperature toughness of a base metal was evaluated only at a temperature of −10° C., it is not clear for a possibility of a new use application in which toughness at a lower temperature is required.
In the technique according to JP '328, since it is necessary that the steel plate has a chemical composition containing increased contents of alloying chemical elements, there is a problem in that there is an increase in material cost and, in addition to that, there is a decrease in toughness in a weld heat affected zone. In addition, the low-temperature toughness of a base metal and a weld heat affected zone are evaluated only at a temperature of −10° C.
In techniques according to JP '839 and 840, although there is an improvement in strain ageing resistance, the low-temperature toughness of a base metal and a weld heat affected zone are evaluated only at a temperature of −10° C.
Moreover, in JP '425, JP '927, JP '027, JP '905, JP '328, JP '839 and JP '840, it is necessary to form a ferrite phase, but the ferrite phase causes a decrease in tensile strength. Therefore, it is necessary to add alloying chemical elements to increase strength to an X60 grade or more according to the API standard, and there is concern that there may be an increase in alloy cost and there may be a decrease in low-temperature toughness.
Therefore, it could be helpful to provide a steel plate having a low yield ratio, high strength and high toughness excellent in terms of strain ageing resistance of an API 5L×70 grade or less which can be manufactured at high productivity, a method of manufacturing the steel plate and a high strength welded steel pipe made of the steel plate.